Repair of Nikkor 24-120 3.5 AF-D That Won't Focus Correctly

[ic-racer]

Here is a Nikkor 24-210 3.5 AF-D it fantastic condition. Came with original Nikon caps and lens hood. The glass is perfect.

However...the lens 'hunts' when zoomed to anything longer than 50mm.

This fault is evident on all camera bodies, thus pointing to a lens issue.

There are NO flexible cables that move when either focusing or zooming. So a break is unlikely.

The electrical contacts are all in good condition. Though they were cleaned with De-Oxit anyway.

Will take it apart to see if it can be fixed.

 

[ic-racer]

Remove the 8 screws indicated in the picture.

 

[ic-racer]

The aperture ring and lens mount will come off leaving this.

The brass semicircle lifts off. In my case it was in triplicate. Other lenses may be different.

 

[ic-racer]

In the case of this lens, the culprit was not that hard to find. This image shows the wipers for decoding the zoom/focal length of the lens.

What has happened here is that the wipers have come off the tracks. The lens was stuck sending only "35mm" to the camera (how to tell? Will explain later...)

So, when the AF electronics in the camera think the lens is 35mm but the lens is zoomed longer than 35mm, it will hunt back and forth and can't find exact focus.

 

[ic-racer]

This is another shot of the wiper assembly. In this case the wipers are too far toward the camera taking the picture. This whole assembly needs to be bent downward to get the wipers back on track.

Ok, but how to do it?

 

[ic-racer]

I was able to come up with this little wrench. I placed it around the screw and bent the metal tab holding the wipers. Just enough!! Not too much.

It worked! The wipers now were riding on tracks and stayed on the tracks as the zoom ring was rotated back and forth.

 

[ic-racer]

How did the wipers jump tracks? I could not determine the mechanism of injury. There was some factory-applied locking agent on the screws I removed to find the problem, so unlikely to be damage from an amateur a 'lens cleaning episode.'

Maybe quality control issue? Or maybe the lens got hot and the track (glued to the lens housing) slipped down a fraction?

Anyway, how can one easily tell if the lens is now sending correct focal length information to the camera body?

I used a Nikon SB-28 flash. With the flash attached to the camera body, and the mode set to TTL/Matrix, the flash will show the lens focal length.

So, before the repair, the flash only indicated "35mm" irrespective of the setting on the zoom ring.

After the repair, not only does the lens focus perfectly throughout the entire zoom range, it will indicate 24, 28, 35, 50, 75, and 80 on the flash. The flash only goes to 80, but the lens focuses perfectly at 120, so I presume it is correctly sending all the focal lengths.

Of note Ken indicated his sample of this lens had some difficulty at some focal lengths, so maybe his lens was manifesting a similar issue with some of the wipers coming off the tracks.

AF errors caused my F100 to give poor accuracy at some focal lengths -- https://www.kenrockwell.com/nikon/24120af.htm

Again, after this repair, my sample focuses instantly and spot-on at all the focal lengths with my F100.

 

[ic-racer]

Here it is, all back together after the repair.